There is growing market demand for handheld and portable electronic devices to include cameras. Mobile phones, personal digital assistants (PDAs), tablets and other portable electronic devices routinely include at least one camera and, in some instances, multiple cameras. There is a consumer demand to increase the presence of cameras in portable electronic devices. Accordingly, there is also a demand to increase the manufacture of camera modules for such devices. Simultaneously, there is incentive to reduce the cost of manufacturing camera modules while improving the yield of the manufacturing process.
In addition, consumer demand requires improved image quality with such cameras. Generally speaking, image quality improves by increasing pixels within image sensors and, with increased pixels, there is a need for an autofocus function associated with the camera module in order to output quality images. Virtually all camera modules used in handheld electronic devices now include an autofocus feature. In general there are two types of auto focusing actuators used in cameras for mobile electronic devices. One is a lens motion type auto focus actuation which utilizes a voice coil motor, piezoelectrics or micro electromechanical system (MEMs) technologies. A second is a lens modification type auto focus actuation which utilizes liquid lens and solid state electro-optical devices. The present invention relates to lens motion type auto focus actuation systems and more specifically those that utilize permanent magnets in connection with the auto focus system such as are used in voice coil motors.
FIG. 1 is a perspective view of a representative example of voice coil motor (VCM) 10 with lens motion type auto focus actuation. FIG. 2 is an exploded view of the VCM 10. In general terms, the VCM 10 includes an axially movable lens 12, a frame member 14, a voice coil motor top spring 16, an electromagnetic interference (EMI) shield 18, a yoke 20 and a base 22. A representative example of a VCM 10 is shown in cross section in FIG. 3. A voice coil motor serves to adjust the position of the lens 12. In general terms, and with respect to adjustment of the lens 12 position, the VCM includes one or more permanent magnets 24 fixed to the yoke 20 and a wire coil 26 associated with the lens housing 28. The coil 26 comprises a number of turns of wire and is positioned radially inwardly from the permanent magnet 24. By driving current through the coil 26, an electromagnetic field is created which interacts with the magnetic field of the permanent magnet 24 to move the lens 12 and drive the lens 12 outwardly or inwardly along its optical axis 30. Changing direction of the current flowing in the coil 26 causes the lens to move in opposite directions. Moving the lens along its optical access 30, towards or away from an image sensor (not shown), focuses a target image on the image sensor. One or more springs 16 are utilized to assist in maintaining the orientation of the lens 12 within the VCM 10 and relative to an image sensor (not shown) and to provide a known resistive or opposing force to the movement imparted by the voice coil motor on the lens 12.
The permanent magnet 24 is generally in the form of a ring or cylinder or may comprise a plurality of arc-shaped magnets which are arranged around the perimeter of the inner wall of the yoke 20. The permanent magnet or magnets generate or create a magnetic flux field that is always present. In contrast, the coil in combination with the yoke also creates a flux field when current flows through the coil. This latter flux field creates what is known as electromagnetic interference (EMI), which may adversely affect nearby or adjacent electrical circuits. The EMI shield 18 is designed to reduce the adverse effects of EMI on surrounding electronics once the VCM is installed in an electronic device and is in operation with current flowing through the coil.
However, during manufacture, the magnetic flux field created by the permanent magnet adversely affects the manufacturing process and, potentially, the acceptable production yield of camera modules containing the VCMs. More specifically, the repelling force or magnetic interference from the permanent magnet(s) inside voice coil motors in adjacent or proximally located voice coil motors or camera assemblies can cause the voice coil motors or camera assemblies to physically shift or move. This is particularly problematic during the manufacturing step of adhering VCMs to image sensors on a printed circuit board or substrate where an adhesive is used to bond these two components to a printed circuit board to create camera modules. Alignment of the VCM relative to the image sensor is a critical step in achieving a camera that outputs acceptable images. If the image sensor and VCM are not properly aligned, the resulting image quality is adversely affected and the camera module formed with the misaligned VCM and image sensor will not pass quality testing and will not be assembled into an electronic device. Thus care must be taken to separate VCMs and associated image sensors a sufficient distance apart from adjacent or proximately positioned VCMs and associated image sensors such that, during the time period before the bonding adhesive fully cures, the repelling force or magnetic interference of nearby permanent magnets does not cause the position of a voice coil motor to shift relative to its associated image sensor. Because of the need for adequate spacing between adjacent or proximate VCMs and associated image sensors, the maximum number of VCMs and associated image sensors that can be processed at one time in any particular manufacturing process is physically limited. Moreover, the existing EMI shields 18 are not designed to resolve this problem, but are designed solely to restrict electromagnetic flux created by the coil and yoke when current is flowing in the coil.